1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device, and more particularly, to a fixing device for use in an image forming apparatus such as a copier, printer or facsimile.
2. Description of the Related Art
A related art image forming apparatus generally includes a fixing device fixing a toner image on a recoding medium such as transfer sheets. The fixing device includes a fixing member (e.g., a fixing roller and a fixing belt) and a pressing member (e.g., a pressure roller and a pressing belt). In the course of a fixing process, for example, the pressing member is heated and is pressed against the fixing member to form a nip therebetween. When a recording medium passes through the nip, a toner image carried thereon is fused, thereby fixing the toner image thereon. The toner generally includes resin and is fused in the nip. Since the toner has a property of adhering to the fixing member when fusing, for example, a releasing agent such as wax is added thereto, and/or the surface of the fixing member is coated with a releasing agent (e.g., silicone oil) so that the toner is unlikely to adhere to the fixing member.
Such a fixing member includes a related-art separation mechanism in the vicinity thereof. The related-art separation mechanism includes a separation tab to forcibly separate the recording medium, which is adhered to the fixing member due to the fused toner image, from the fixing member.
In this regard, the separation tab of the related-art separation mechanism slides in contact with the fixing member, and therefore the toner tends to accumulate on the separation tab. The accumulated toner soils the recording medium. In addition, a sliding mark can be generated on the fixing member. The sliding mark not only shortens the life of the fixing member but also produces an abnormal image on the recording medium.
Recently, the surface of the fixing member is rarely coated with a releasing agent due to handling difficulty, resulting in increasing the likelihood of generating the accumulated toner problem and the sliding mark problem. Therefore, a variety of proposals have been made in attempting to solve the above-described problems.
One example attempts to use a separation plate (referred to as a non-contact separation plate) separating a recording medium from a fixing member without being contacted therewith.
Another example attempts to use a separation guide member (referred to as a non-contact separation guide member) that is integrally formed of synthetic resin and separates a recording medium from a rotation member acting as a fixing member without being contacted therewith. In this example, the edge of the separation guide member is positioned based on the position of a bearing of the rotation member. Specifically, a support member supporting the separation guide member abuts the bearing to position the separation guiding member with respect to the rotation member.
In addition to solving the above-described problems, a fixing device of recent years is expected to have reduced size and greater serviceability. In such a fixing device, a fixing roller including an elastic member such as sponge members made of silicone foam is used to increase the degree of contact with a pressure roller, thereby increasing a nip width to meet such recent demand. The fixing roller including such sponge member can also reduce start-up time and electric power consumption of the fixing device.
However, the fixing roller with such sponge member (e.g., the silicone foam) has a short life due to the increased degree of contact with the pressure roller. In attempting to solve this problem, a release mechanism is disposed to release the pressure of the pressure roller against the fixing roller, for example, when the fixing process ends and the fixing device is not in operation.
In the above-mentioned fixing devices, the pressure roller contacts the fixing roller in a nip. In other words, the pressure roller digs slightly into the fixing roller in the nip when pressure is applied from the pressure roller to the fixing roller. When the release mechanism releases the pressure of the pressure roller, the fixing roller is released from such pressure and the surface thereof returns to an initial state. In the course of such pressure release, the surface of the fixing roller expands.
When the surface of the fixing roller expands, a non-contact separation member such as the non-contact separation plate and the non-contact separation guiding member above can damage the surface of the fixing roller. In order to avoid this problem, the edge of non-contact separation member has to be set to a position at which the edge is not contacted with the fixing member even when the surface of the fixing member expands. In this case, however, the separation member does not have good separability.
In the first-mentioned example, the non-contact separation plate includes an abutting member to control a gap between an edge thereof and a fixing member (i.e., a rotation member). Such an abutting member needs to have a thickness of at least 1 mm. Consequently, the distance between a point at which the abutting member contacts the fixing member and the edge of the separation plate should be long (about 6 mm) when the edge of the separation plate is disposed in an immediate vicinity of a downstream side from a nip formed between the fixing member and a pressing member.
In this regard, the edge of the separation plate cannot respond well to the expansion of the surface of the fixing member, thereby increasing the risk of damaging the surface thereof. In addition, the gap cannot be reduced to a desirable level.
In the second-mentioned example, the edge of the non-contact separation guiding member having a specific curvature radius is positioned based on the position of the bearing of the rotation member (i.e., the fixing member). Consequently, the edge of the non-contact separation guiding member cannot respond to the expansion of a surface of the rotation member in an immediate vicinity of a downstream side from a nip formed between the rotation member and a pressing member. Further, it is relatively difficult to position the edge of the separation guiding member in the immediate vicinity of the downstream side from the nip in which the surface of the rotation member can expand due to pressure applied from the pressing member. Therefore, the gap between the edge of the separation guiding member and the surface of the rotation member cannot be reduced to a desirable level.